Living anionic polymers are well known in the art. In these systems, the formation of active polymer chain centers during polymerization is faster than chain propagation and the centers remain active until all of the monomer has been consumed. Coupling of such living anionic polymers has been disclosed wherein stable polymeric carbanions are reacted with certain electrophilic species, such as phosgene or dichlorodimethylsilane. This coupling process has been used, for example, in the preparation of telechelic polymers and ABA triblock copolymers.
By contrast, relatively few truly living cationic systems are known and coupling of the living cationic polymers has not been studied to a great extent. Thus, for example, coupling agents for living cationic poly(vinyl ether) and poly (.alpha.-methylstyrene) have been reported, but high coupling efficiency was only obtained when the number average degree of polymerization (DP.sub.n) was quite low (i.e., .about.10) (see articles by Fukui et al. in J. Polym. Sci., Part A: Polym.Chem., 31, 1531(1993); Macromolecules, 26, 7315, (1993); Macromolecules, 29, 1862 (1996)). Sodiomalonate anions, which were used as coupling agents for living poly(isobutyl vinyl ether), also had drawbacks due to their limited solubility in typical polymerization solvents and use thereof resulted in a low yield for the coupling reaction (Fukui et al. in Polym.Chem., 31, 1531). Non-ionic coupling agents, such as bifunctional silyl enol ethers were successfully employed in the coupling of short living chains (DP.sub.n .about.10) of poly(isobutyl vinyl ether) and displayed high coupling efficiency (i.e., percent of active centers coupled &gt;95%). However, the silyl enol ether is very expensive and this method is not commercially viable, especially for low molecular weight polymers.
In-situ coupling of living polyisobutylene (PIB) has recently been disclosed using bis-diphenylethylenes (bis-DPEs) such as 2,2-bis{4-(1-phenylethenyl)phenyl}propane and 2,2-bis{4-(1-tolylethenyl)phenyl}propane where the two DPE moieties are separated by a spacer group. While the relative amount of coupling agent is negligible in the coupling reaction of high molecular weight PIB, the above process is impractical for the coupling of oligomeric PIB (e.g., number average molecular weight, M.sub.n, of about 1000 to 5000) since the relative amount of the expensive coupling agent would be too high (e.g., in the range of about 5 to 15% based on the weight of the coupled product) (see Bae et al. in Macromolecules, 30, 198, (1997)).
A facile route has also been developed for the quantitative coupling of oligomeric polyisobutylene (PIB) through c)isopropenyl functionality (--CH.sub.2 C(CH.sub.3).dbd.CH.sub.2) with catalytic amounts of triflic acid (CF.sub.3 SO.sub.3 H) in hexanes at -80.degree. C. (see Coca et al. in Macromolecules, 30, 649 (1997)). However this process requires an additional step, namely dehydrohalogenation, to obtain the (.omega.-isopropenyl functional PIB.
Additionally, U.S. Pat. No. 5,260,378 to Gandini et al. teaches the use of furan derivatives as a binding group (b) in the formation of A-b-B block copolymers. In a typical procedure, a solution of monomer A and the furan derivative is reacted in the presence of a Lewis acid to polymerize the monomer. Excess monomer A is then stripped out and monomer B is added. After polymerization of B, the system is quenched with an alcohol to produce the A-b-B structure. Thus, although the Gandini et al. patent teaches the use of certain furan derivatives as a binding group in a complex polymerization scheme to prepare copolymers having two different polymer blocks, there is no suggestion in this reference to couple an essentially identical living cationic polymer. Moreover, Gandini et al. contemplate the use of furan derivatives having only one furan ring (e.g., their formula (I) wherein x=p=m=n=0). When such a compound was used in an attempt to couple a cationic living polyisobutylene no coupling was observed (i.e., the molecular weight remained essentially the same as that of the uncoupled living polymer).
Thus, there is a need for an efficient and inexpensive agent which allows in-situ coupling of essentially identical living cationic polymers.